1. Field of the Invention
The present invention relates to a nuclear magnetic resonance imaging, and more particularly, to a nuclear magnetic resonance imaging for obtaining SAS (surface anatomy scan) and angiographic images.
2. Description of the Background Art
In general, a nuclear magnetic resonance image is obtained by detecting nuclear magnetic resonance signals in terms of complex signals by using a quadrature detector, and then applying the complex Fourier transform to the detected complex signals in order to obtain complex image data I=I.sub.r +jI.sub.j.
Conventionally, the nuclear magnetic resonance image to be displayed on a display unit is then obtained from the complex image data I as an absolute value display .vertline.I.vertline.=.sqroot.I.sub.r.sup.2 +I.sub.j.sup.2, a phase corrected real image display I.sub.r, a phase corrected imaginary image display I.sub.j, or an argument image .angle.I=Tan.sup.-1 (I.sub.j /I.sub.r).
Among these displays, the absolute value display has a drawback that an additional processing time is required for carrying out a square root calculation. This can be a practically unnecessary disadvantage when the diagnosis using displays does not require the absolute value display.
In a case of obtaining the SAS image for showing a brain surface structure, first a spin echo multi-slice imaging is performed such that nuclear magnetic resonance echo signals for usual diagnostic images such as density emphasized images and T.sub.2 emphasized images as well as echo signals for fat suppressed and cerebrospinal fluid emphasized images are obtained, and then the SAS image is obtained by applying a weighted sum calculation to the spin echo multi-slice images.
Here, the echo time TE for the spin echo multi-slice imaging have to be as long as 250 msec which is much longer than a case of obtaining the usual density emphasized image in which the required echo time is 20 msec or a case of obtaining the usual T.sub.2 emphasized image in which the required echo time is 120 msec. This is because it is necessary in a case of obtaining the SAS image to enhance the contrast between the brain body and cerebrospinal fluid by collecting sufficiently larger amount of the nuclear magnetic resonance signals from the cerebrospinal fluid in the vadum which contains information on the brain surface structure than the nuclear magnetic resonance signals from the brain body. The reduction of the echo time results in the lowering of the contrast and the deterioration of the three dimensional quality of the SAS image.
This feature of the SAS image taking gives rise to a drawback that a number of slices that can be taken in a given echo pulse repetition time TR is smaller in a case of SAS image compared with other cases of usual images, so that in order to take the same number of slices a case of SAS image requires much longer period of time than cases of usual images, which is not desirable from a point of view of a safety of the patient.
As a solution to this problem, there is a method of using the multi-slice images taken with a shorter echo time TE such as 120 msec, and obtaining the three-dimensional quality by forming stereographic SAS images in which the weight sum calculation is varied and displaying the stereographic SAS images in a stereographic mode or an animation mode.
However, in the SAS image obtained by this method, the three dimensional quality of the SAS image is poorer than the SAS image obtained by the multi-slice images taken with a longer echo time TE such as 250 msec.